1. Field of the Invention
The present invention relates to an apparatus and method for fabricating lightweight structural panels having a corrugated core structure. The present invention also relates to a structural panel fabricated from thermoplastic sheet stock of indefinite length using the apparatus and method of the present invention.
2. Background of Related Art
Structural panels made of thermoplastic materials or glass-reinforced thermoplastic materials and having expanded core sections have been experiencing increased use in the automotive/trucking industry as well as in the building and construction industry. This type of structural panel exhibits the desirable characteristic of providing high strength at low weight, not readily available when using various other conventional materials. Thermoplastic or glass-reinforced thermoplastic (thermoplastic-composite) materials also provide the additional advantage of increased resistance to corrosion.
Methods for forming structural panels are known which employ thermoplastic sheets of fixed length and width, the maximum dimensions of which are dictated by the size of the mold plates and platens used therein. Such methods can be characterized as discrete operations in the sense that the apparatus runs through a predetermined cycle for each panel formed, and no capability is provided for continuous processing.
Continuous-type processes for forming thermoplastic panels having corrugated core structures are also known. In one such known process, a core sheet may be passed between a pair of corrugation-forming drums shortly after the sheet has been extruded. The contacting and bonding of the outer sheets occurs nearly immediately after the extrusion of the outer sheets such that the bonded sheets must follow the processing path around the circumference of the drums, the structure finally being formed into a flat panel at a later stage in the processing. Such a process suffers from the disadvantage that the core sheet is moved around several drums and is bent in two different directions after the corrugation is formed. Such bending induces undesirable stresses and distortions in the corrugations.
In another known continuous-type process, two outer sheets are bonded to a corrugated core sheet in which the upper and lower crests of the corrugations are formed in a direction transverse to the axial processing direction of the machine. A lower outer sheet is bonded to the core sheet by way of a relatively sophisticated and complex endless loop conveyor having a series of heater-bearing projections which are synchronized with the travel of the core sheet in a manner such that the projections will mate with the lower crests of the transverse corrugations to provide the heat and pressure necessary to effect the bonding. A top outer sheet is bonded to the core sheet by heating the outer surface of the top sheet with heated rollers and applying pressure between the outer sheet and the upper crests of the corrugations.
Such a process suffers the disadvantage that the method of bonding the top sheet to the core sheet is likely to induce distortions in the surface as the heat is applied from above the top sheet, thereby heating the exposed surface. Further, such a process requires a relatively complex machine.
It is therefore an object of the present invention to provide an apparatus having a continuous processing capability for making thermoplastic or thermoplastic-composite panels of indefinite length.
It is a further object of the present invention to provide an improved method of fabricating a structural panel of indefinite length which can be used with monolithic sheets made of either thermoplastic or thermoplastic-composite materials.
Another object of the present invention is to provide a high strength, low weight structural panel having a corrugated core sheet and an outer skin sheet, at least one of the sheets being a thermoplastic-composite or glass reinforced sheet.
Yet another object of the present invention is to provide an improved apparatus and method for fabricating a structural panel which produces a uniform corrugated structure having an outer skin sheet or sheets in a single automated continuous process.
It is a further object of the present invention to provide a structural panel, and an apparatus and method for producing said panel, having at least one outer skin sheet which is substantially free from surface distortion and is of a suitable quality surface finish and appearance whereby paint may be applied to the surface and whereby the panel can be used as an exposed surface in automobile, trucking or other applications requiring a glassy smooth surface finish.